Thursday, January 24, 2013

The rise of the Isthmus of Panama created the conditions with
which to test hypotheses considering the causes and consequences of
adaptive radiation and speciation of terrestrial, freshwater and marine
species. The rise of the isthmus first led to an important
paleozoogeographic event called the “Great American Interchange”
characterized by the migration of terrestrial and freshwater fauna
between South American and North American ecozones. In addition, the
Isthmus of Panama isolated reproductive populations of marine species
across taxa that allowed the speciation and adaptive radiation of
geminate, or sister, species on either side of the isthmus to different
biotic and abiotic conditions in the western Atlantic and eastern
Pacific environments.

The evolution of Atlantic
and Pacific sister species has also been the subject of decades worth of
research that has measured rates of evolutionary change on the
molecular level by calibrating the molecular clock with the geologic
data on the dates of the rise of the isthmus of Panama, which is thought
to have occurred about 3.5 million years ago, although this hypothesis
is now contested, as an alternative, but not conclusive, date of
approximately 18-20 million years has been proposed for the rise of the
isthmus. Nevertheless, the rise of the isthmus has permitted one of the
most important and extensively studied natural experiments in evolution,
a natural experiment that now allows scientists to test specific
predictions considering the ecological and evolutionary dynamics that
have allowed species to adapt to the biotic and abiotic conditions in
different environments.

Adaptation to different
environment conditions can impose differential selection pressures on
reproductively isolated populations. Therefore, we should predict that
physiological and behavioral traits should confer adaptive value to the
different conditions found in the coastal marine environments of Panama.
We have recently tested the effects of adaptive radiation to different
environmental conditions of the Pacific and Atlantic Oceans on the
strategies of larval development in two geminate species of coastal mud
snails (Potamididae) across the Isthmus of Panama, Cerithideopsis
californica on the Pacific coast and C. pliculosa on the Atlantic.
Differences in the productivity of western Atlantic and eastern Pacific
coastal environments in this region allowed us to predict that in an
environment of relatively low productivity (the Atlantic), natural
selection will favor the evolution of larval developmental strategies
that allow it to survive in these environments of low productivity
through increased maternal investment and reduced larval duration in
development.

This pattern of larval development is consistent with models of
life-history evolution in which increased maternal investment is
selected for in low-productivity environments. In this case, we observed
larger size and reduced planktonic duration of larvae in the Atlantic
than in a relatively highly productive environment (the Pacific). While
geminate sister species pairs across taxa that inhabit the Pacific and
Atlantic coasts of Panama are excellent systems with with to study
adaptive radiation to different environments, this pair of snail species
is an especially important system with chih to study the drivers of
life-history evolution because both species have an exceptionally broad
biogeographical distribution, spanning over 30 degrees latitude on both
the Atlantic and Pacific coasts. Both of these species are found in
habitats with different abiotic and biotic conditions, such as food
availability, salinity and temperature that change with increasing
latitude that can thus impose differential selection on larval and adult
snails that can drive the evolution of important life-history traits,
as was observed in the study considering larval developmental strategies
across the Atlantic and Pacific coasts of Panama. Testing whether
differences in life history traits of Atlantic and Pacific snails across their latitudinal range is an evolutionary
or plastic response to different environments, however, is a whole
different story. Write on!

Paper can be found here. http://mollus.oxfordjournals.org/content/77/3/255.short

Wednesday, January 16, 2013

Biodiversity is higher in the tropics. Terrestrial productivity
is higher in the tropics. The pace of life is faster in the tropics. Mountain
passes are higher in the tropics. The tropics are just bigger, faster, and
stronger. So what about eco-evolutionary dynamics? Are they stronger in the
tropics? A recent trip to Panama provided the motivation to speculate on this
possibility.

Coati

Early
in the trip, I visited the famous research site of Barro Colorado Island (BCI),
where I was able – with my family – to see howler monkeys and all sorts of
other wonders. Back at the town of Gamboa a few days later, I was called on to
give a lecture to the “tropical boot camp” class that included graduate students
from the McGill-STRI NEO program, the STRI-Indiana IGERT program, and Arizona
State University. I decided to give my boiler-plate talk outlining a conceptual
framework for eco-evolutionary dynamics because I figured most of the students would
be ecologists and it would perhaps be worthwhile to encourage them to include
an evolutionary perspective into their ecological thinking.

A leaf cutter ant in the clutches of an ant lion (zoom in for a better view of the lion).

During the lecture, I set up several key questions facing
the study of eco-evolutionary dynamics, one being the importance of evolution (e.g.,
changes in phenotypic traits) relative to other non-evolutionary ecological
forces (e.g., precipitation, temperature, flooding) in shaping ecological
dynamics at the population, community, and ecosystem levels. The standard work
addressing this question at the population level is that comparing the effects
of phenotypic traits (e.g., body mass) on the population growth rate of
ungulates in Canada and Scotland in comparison to climate variables
(e.g., rainfall, Pacific Decadal Oscillation). Remarkably, effects of the two causal forces (evolution versus ecology) are roughly the same in each study population,
suggesting that phenotypic change is incredibly important to ecological dynamics.
Halfway through explaining all of this, it began to strike me as silly to use
an example from a temperature vertebrate while lecturing in a building situated on the borders of a verdant tropical rainforest. Why not use a tropical example
– even if just for hypothetical illustration.

Lounging capybaras

For some reason, my mind hit on howler monkeys. What effect,
I posed as an example, would evolutionary changes in the phenotypes of howler
monkeys have on the productivity or diversity of the BCI forest relative to the
amount of rainfall. I had no idea of the answer, of course, which got me to
wondering. Would eco-evolutionary dynamics be stronger or weaker in the
tropics? It seems like an opportune time for some speculation.

Millipede delight

Several properties might increase the strength of eco-evolutionary
dynamics. (1) Faster rates of phenotypic change. Perhaps the tropics have faster
rates owing to the more rapid pace of life, or perhaps not given their more
stable environment. (2) When the species causing the ecological effects have
large effects as individuals, such as in the case of keystone species. Perhaps the
tropics have more of these (elephants!), or perhaps not given that many more
species are present and so the effect of any single species (besides
elephants) might be weaker. (3) When the species causing the ecological effects
are very numerous (bacteria, viruses, and some insects and plants).
The tropics likely have more such organisms given the overall greater
productivity, or perhaps not given that so many species are present the effects
of any one species – however numerous – might be swamped by all the other
numerous species. (4) When feedbacks between ecology and evolution are
stronger, such as when trait changes causes an ecological change that promotes
(through selection) further changes in that trait. Perhaps the tropics have
more feedbacks of this sort because the environment is not reset each year by
winter and because so many cool mutualisms are present, or perhaps not because
the system can be reset by dry and wet seasons and because mutualisms in
temperate regions might have stronger effects given the relative paucity of
other species. So would we expect eco-evolutionary dynamics to be stronger or
weaker in the tropics than in temperature regions given that effects seem to point in both directions in each case?

Yummy dung

I suggest that evolutionary dynamics on the
part of single species (i.e., effects of the evolution of a focal species on aggregate ecological variables) might be weaker in the tropics – simply because the countless other species dilute the effects of any one species. However, I also suggest that eco-evolutionary dynamics in aggregate (i.e., across
all species) will be stronger in the tropics – because there are so many other
species and interactions, because they have been for around longer, and because
the environment is somewhat more stable. I also suggest that
eco-evolutionary dynamics associated with phenotypic CHANGE might weaker in the
tropics given that organisms have had more time to stabilize their adaptations
and so might be less subject to contemporary phenotypic change. However, I also suggest that eco-evolutionary dynamics based on phenotypic STABILITY
might be stronger in the tropics. By this I mean that the very stability seen
in (some) tropical ecosystems is likely the result of continual ongoing evolutionary
change. That is, so many interacting species are present that extinction and extirpation
would be common were it not for constant, ongoing eco-evolutionary dynamics
that maintain and improve adaptations and thereby stabilize population sizes.

Spooning is universal

Of course, this is all speculation provided in fun and on
the fly but how can one not be motivated to think about the uniqueness of the
tropics when watching your children feed leaf cutter ants to ant lions, go all
warm and fuzzy over a howler monkey mom and baby spooning, marvel at a massive
tarantula, try to get close to a capybara, watch dung beetles fight over a prime piece of monkey stink, sneak up on a group of foraging
coatis, chase a praying mantis around and around a tree, and try to find toads
that look like leaves.

Sunday, January 6, 2013

As many of you may know, a lot of research is done using guppies and their ectoparasite Gyrodactylus turnbulli as a model system for ecological and epidemic dynamics:

Gyrodactylus on a fish. Video credit: Christina Gheorghiu.

The really cool thing about these parasites is that they’re easy to observe and to quantify, so we can collect data on their presence and growth-rates over time without having to sacrifice the host (guppy). Also they reproduce exponentially and are transmitted directly via host-host contact thus causing epidemics in the wild, a characteristic atypical of most macroparasites. These two characteristics combined make G. turnbulli a very convenient model parasite for studying epidemic dynamics, as has been done here, here, and here. Epidemics and high parasite loads (sometimes reaching well over 300 parasites per fish) lead to high levels of guppy mortality, making this system also well suited for the study of host-parasite coevolution.

Body condition (a metric of individual weight to length ratio) is used as a common proxy for health or well being. While research on how an individual’s food intake can affect their overall body condition and disease resistance is abundant, the question of how these relationships might translate to the population scale remained unanswered. Thus we decided to investigate how food availability in the environment and host body condition relative to others in the population affected the overall outcomes of an epidemic, using the guppy-Gyrodactylus system as a model. To answer this question, we set up laboratory populations of guppies and subjected them to different levels of food availability and measured their relative body conditions (i.e. how much better or worse-off were they relative to all other experimental fish). Then we introduced parasites to the populations by infecting one randomly chosen fish from each group and monitored the epidemics over time. Pretty simple and it gives a huge load of information.

If you want to read more about our methods, measurements and analyses, you can do so here. Below we provide an overview of our most interesting results.

The first cool thing that we found is that in general, host relative body condition was positively associated with parasitism. This would in principle mean that the healthier a fish is, the more parasites it will have; nonetheless it can also mean that the “fatter” a fish is for its length, the more likely it is to have parasites, and when it is infected, the more likely it is to have more parasites. Specifically, we found that the incidence of parasitism (the number of fish that became infected over the course of our experiment) was greater in populations with a higher average condition index. This is pretty counter-intuitive, and it was therefore assumed that those with a high relative condition index would be more resistant to disease.

Another interesting finding was that the relative condition of the fish we used to introduce the disease to the population (the “source” fish) mattered a lot. Almost all of our epidemic variables were significantly affected by this factor. Specifically we found that the peak burden of parasites in the populations was significantly positively impacted by the relative condition index of the “source” fish. Moreover, we found that parasites were more aggregated (crowded on one particular fish) when the condition of the source fish was high, and that this crowding usually occurred on high condition source fish, particularly when the average condition of the population was low. This is exciting because these results indicate that the way in which a disease is introduced to a population, or rather, the characteristics of the host through which it is introduced, can have significant impacts on the course of the epidemic (both the burden and distribution of parasites) in the population as a whole. To our knowledge, this is a novel result that could inspire further investigation.

So why would higher condition fish have more parasites? One theory is that larger fish are so because they invest more energy into growth rather than into defense against disease, making them more susceptible to parasite infection. It is also possible that larger fish simply make better hosts because they provide more resources, allowing the parasites to rapidly grow and reproduce while making them less likely to transfer to a host of lower quality. This idea seems to fit our result that parasites were strongly aggregated on hosts of high condition.

But what about food availability? While our results did not indicate any significant impacts of food availability alone, we did find that the interaction of food availability with the condition of the source fish negatively impacted our epidemic parameters. What this means is that the positive relationship we found between peak parasite burden/aggregation and source condition was dampened (the slope of the regression is not as steep) as food availability increased. We think this means that in populations with high food availability, fish may be able to consume more resources and dedicate that energy towards resistance, rather than fat storage or growth, thus decreasing their “quality” as a host causing parasites to grow at a slower rate and forcing them to disperse throughout the population in search of a better host.

Overall, our results present some new and interesting ideas which we hope to follow-up on in future investigations.